The background art of the present invention will be described using an excavator as an example.
For example, as shown in FIG. 5, a general excavator comprises a crawler-type base carrier 1, an upper slewing body 2 mounted on the base carrier 1 so as to be slewed around an axis X that is perpendicular to the ground, and an excavating attachment 3 attached to the upper slewing body 2. The excavating attachment 3 includes a boom 4 capable of being raised and lowered, an arm 5 attached to a tip of the boom 4, a bucket 6 attached to a tip of the arm 5, and a plurality of cylinders (hydraulic cylinders) for actuating the boom 4, the arm 5, and the bucket 6, respectively, namely: a boom cylinder 7, an arm cylinder 8, and a bucket cylinder 9.
Japanese Patent Application Laid-open No. 2010-65510 (Patent Document 1) discloses an excavator such as that described above, the excavator comprising: a hydraulic motor for slewing an upper slewing body; a slewing electric motor connected to the hydraulic motor; a direct-communication selector valve capable of bringing respective pipe-lines on both sides of the motor connected to a pair of ports of the hydraulic motor, respectively, into direct communication with each other; and an electric storage device, wherein the direct-communication selector valve, during deceleration of the rotation, returns hydraulic fluid discharged from the motor to a inlet side of the motor and the slewing electric motor performs a generator action to produce regenerative power, the electric storage device storing the regenerative power. With this technique, the direct-communication selector valve lowers back pressure acting on a motor outlet side during rotation deceleration to reduce drag load on the hydraulic motor, thereby enabling efficiency of recovery (that is, regeneration) of inertial kinetic energy to be improved. There is provided a hydraulic brake device including a pair of relief valves between the pipe-lines on both sides of the motor; however, the hydraulic brake device is not operated during rotation deceleration but only performs a stop holding function immediately after slewing is stopped.
This technique, though improving regeneration efficiency during rotation deceleration, has a problem that regeneration efficiency of slewing energy is still insufficient because no regenerative action is produced in a driving for slewing, that is, in acceleration including start-up or in a steady operation. In addition, the direct-communication selector valve, which is set at an open position during driving for slewing and switched to a direct-communication position during regeneration, i.e., during deceleration, has a further problem of causing a large fluctuation in pressure at the moment of being switched to thereby deteriorate operability.
Patent Document 1: Japanese Patent Application Laid-open No. 2010-65510